PCI: mvebu: Only remap I/O space if configured

If there is no PCI I/O aperture configured in the Device Tree, it does
not make sense to create the virtual mapping for the PCI I/O space,
since we will anyway not create the MBus window that will allow to
access it. Therefore, do the pci_ioremap_io() only if necessary.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>